The purpose of this project is two-fold: (1) to understand the parameters and mechanisms operating in a realistic model system for exocytosis, with particular emphasis on its relevance to transmitter release at synapses; (2) to reconstitute and study, in lipid bilayer membranes, ion permeability systems existing in biological membranes. The system to be studied is the fusion of phospholipid vesicles with planar phospholipid bilayer membranes as evidence by (a) the incorporation into the planar membrane of an intrinsic membrane protein originally present in the vesicle membrane and (b) the discharge of vesicular contents across the planar membrane. The basic issues are to understand quantitatively how osmotic gradients across the planar membrane drive vesicle-membrane fusion and how micromolar concentrations of Ca ions catalyze these processes when a calcium-binding protein is incorporated into the planar and vesicular membranes. These issues bear directly on the role of Ca ions in stimulating transmitter release at presynaptic terminals (e.g., the neuromuscular junction) and in stimulating exocytosis from such cells as the chromaffin cells of the adrenal medulla and the islet cells of the pancreas. In addition, by fusing with the planar membrane vesicles containing naturally occurring ion-conducting channels (e.g., the sodium channels of toad urinary bladder or the cholinergic channels of electroplax), these channels can be studied by classical electrophysiological techniques in a particularly simple system. Furthermore, this system offers a unique and sensitive assay for these channels during and after isolation and purification procedures.